Hot-dip galvanized iron and steel materials, coated by dipping in molten zinc, are used for corrosion protection purposes in a wide range of applications, forming parts and facilities in the fields: of building and construction, civil engineering, agriculture, fisheries, chemical plants, electric power supply and communications, and so forth.
For pylons and other towers, lighting poles, guard rails, temporary stands and frames for various operations and displays, shells and planks, and the like, there has been growing demand in recent years for colored hot-dip galvanized materials that present attractive appearances matching the environments involved, in preference to the classic hot-dip galvanized steels with metallic luster. With the spread of the aesthetic sense the colored hot-dip galvanized articles show promise, with extensive potential demand in architecture, civil engineering, industrial plants, electric power supply and communications, transportation, agriculture, marine products and other industries.
Coloration of hot-dip galvanized steels has usually been by the application of paints. The method has the disadvantage of the paint film eventually coming off the coated surface. This results from the activity of Zn in the coating of the hot-dip galvanized steel that causes gradual alkali decomposition of the fatty acid constituting the oily matter in the paint, leading to the formation of zinc soap that hampers the adhesion of the paint film to the underlying surface.
In an effort to eliminate the disadvantage, a complex procedure has had to be followed. An iron or steel article is first galvanized by dipping it into a molten zinc bath. The coated article is exposed to the air for one to three weeks so that corrosion products such as Zn(OH).sub.2, ZnO, ZnCO.sub.3, ZnCl.sub.2 and the like deposit on the coated steel surface. The surface is then cleaned and colored.
Aside from the coating method described above, another approach that depends on the color-developing action of the oxide film in the hot-dip galvanizing is known in the art. For example, Japanese Patent Application Publication No. 42007/1971 discloses a coloring treatment that uses a coating bath prepared by adding at lest one element selected from the group consisting of titanium, manganese, vanadium and the like to a hot-dip galvanizing bath. However, the hot-dip galvanized coatings obtained by the disclosed technique have been found to be generally very thin and light, with tendencies of rapid color fading and film separation with time. The desired color development is difficult to control precisely, often bringing out dim, indefinite hues.
For such reasons, even though many years have lapsed since its development, hot-dip galvanized coloring technique has not been put into practical use on a commercial scale.
Under such circumstances, there is a steady demand in the art for many improvements such as:
(a) The development of new colorings which have not yet obtained in the past; PA0 (b) The development of colors which are more beautiful and clearer than ones previously obtained; PA0 (c) The enhanced stability of color development; PA0 (d) The development of a coloring system wherein the inherent corrosion resistance of galvanized zinc coating is not sacrificed; PA0 (e) The development of a coloring system wherein there is a lesser degree of color change with the lapse of time; and PA0 (f) The development of a coloring system which has an easy and stable operation.